better debug info for a mismatch

compiler/types/src/subs.rs

@@ -327,6 +327,14 @@ fn subs_fmt_desc(this: &Descriptor, subs: &Subs, f: &mut fmt::Formatter) -> fmt:
     write!(f, " m: {:?}", &this.mark)
 }
 
+pub struct SubsFmtContent<'a>(pub &'a Content, pub &'a Subs);
+
+impl<'a> fmt::Debug for SubsFmtContent<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        subs_fmt_content(self.0, self.1, f)
+    }
+}
+
 fn subs_fmt_content(this: &Content, subs: &Subs, f: &mut fmt::Formatter) -> fmt::Result {
     match this {
         Content::FlexVar(name) => write!(f, "Flex({:?})", name),
@@ -345,6 +353,14 @@ fn subs_fmt_content(this: &Content, subs: &Subs, f: &mut fmt::Formatter) -> fmt:
     }
 }
 
+pub struct SubsFmtFlatType<'a>(pub &'a FlatType, pub &'a Subs);
+
+impl<'a> fmt::Debug for SubsFmtFlatType<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        subs_fmt_flat_type(self.0, self.1, f)
+    }
+}
+
 fn subs_fmt_flat_type(this: &FlatType, subs: &Subs, f: &mut fmt::Formatter) -> fmt::Result {
     match this {
         FlatType::Apply(name, arguments) => {

compiler/unify/src/unify.rs

@@ -1076,8 +1076,8 @@ fn unify_flat_type(
             // any other combination is a mismatch
             mismatch!(
                 "Trying to unify two flat types that are incompatible: {:?} ~ {:?}",
-                other1,
-                other2
+                roc_types::subs::SubsFmtFlatType(other1, subs),
+                roc_types::subs::SubsFmtFlatType(other2, subs)
             )
         }
     }